Bath Overflow drain Gasket issue

I'm installing a new Americast 5'-0" tub with a Watts brass tub drain assembly.
everything was moving smoothly until I went to install the tapered washer on the backside of the tub overflow.

I understand that most of the time the thick side of the washer goes down. In this case, I cannot get a tight seal between the tub and the overflow flange with the supplied tapered washer. IF the bottom is tight, there is a gap at the top. It appears that the angle of the tub and the angle of the overflow flange is nearly identical. I cannot find a thick flat washer locally for this application.

Turn the tapered washer sidewise. I install them that way all the time in that situation. All you're looking for is a tight seal all the way around. There are tubs that need the taper at the bottom, but certainly not all of them.